1. Field of the Invention
This invention is directed to a process for the selective production of 4,4'-alkylidene diphenyl diisocyanates and specifically 4,4'-methylene diphenyl diisocyanate, to the exclusion of other isomers and oligomers.
2. Description of the Prior Art
Organic isocyanates are materials of commerce employed in the production of polyurethane resins. Of increasing commercial importance are the polymethylene polyphenyl isocyanates, referred to as PMPPI. One member of the PMPPI family, 4,4'-methylene diphenyl diisocyanate, referred to in the industry as 4,4'-MDI, is especially desirable because it exhibits outstanding properties in polyurethanes.
The prior art with respect to production of isocyanates, both as to methods for their chemical synthesis and as to their commercial production, is summarized in Kirk-Othmer, "Encyclopedia of Chemical Technology", Second Edition, Volume 12, pages 53 et seq. Under the heading of Manufacture, it is stated that:
"The reaction of amines with phosgene, commonly referred to as phosgenation, is, for economic reasons, used almost exclusively for the manufacture of isocyanates . . . Because of the several side reactions and associated complications, the development of practical, high-yield reaction conditions has been studied extensively and many patents have been issued covering detailed procedures . . .
"The problems are multiplied in the manufacture of a diisocyanate, where the simple by-products may be intermolecular, e.g., a mixed carbamyl chloride/amine hydrochloride, and the urea by-product may be polymeric.
"It appears that all commercial manufacturing processes for aromatic isocyanates have the following approach: (1) The solution of an amine in an aromatic solvent . . . is mixed with a solution of phosgene in the same solvent at a temperature below 60.degree.; (2) the resulting reaction mixture slurry is then digested in one to three stages for several hours at progressively increasing temperatures up to 200.degree. C. with the injection of additional phosgene; and (3) the final solution of reaction products is fractionated to recover hydrogen chloride, unreacted phosgene and solvent for recycling, isocyanate product, and distillation residue for incineration."
In the conventional production of 4,4'-MDI, the amine which is phosgenated, 4,4'-methylene dianiline (4,4'-MDA), is produced by the condensation of aniline and formaldehyde. This reaction is not highly selective to 4,4'-MDA but leads unavoidably to the production of a mixture of dimers, trimers and higher oligomers, including dimers of other than the 4,4'-product.
A different process for the production of polymeric diisocyanates was recently announced by Atlantic Richfield Company. As described in Chemical Week, Nov. 9, 1977, pp. 57-58, the process comprises the steps of reacting nitrobenzene, carbon monoxide and an alcohol to form corresponding urethanes (alkyl phenyl carbamates). The reaction product is reacted with formaldehyde to produce a condensate which contains p,p'-methylene diphenyl dialkylcarbamate and higher oligomers. That product is, in turn, thermally split into the corresponding "polymeric diisocyanates" and alcohol, which is recycled. The set of reactions is reported to involve the use of high temperatures in the range between 100.degree. and 200.degree. C. in the first reaction step and between 200.degree. and 300.degree. C. in the decomposition step, and the reaction is reported to be non-selective, leading to a mixture of polymeric diisocyanates.
I am not aware of any process known in the prior art which is highly selective for the production of 4,4-MDI or its alkylidene homologs, nor of any cyclic process for selective production of 4,4'-MDI from formaldehyde and phenyl isocyanate such as that which is the subject of the present invention.
After the present process was developed, the following references were found in a search for disclosures related to the process: